Nephelometer instruments for measuring the mass per unit volume of ambient aerosols have historically used a light source, such as a diode laser or light-emitting diode (LED), to illuminate a view volume containing particles suspended in a gas carrier. The amount of detected optical scatter from the illuminated particles in the view volume has then been taken as a measure indicative of the amount (mass) of particulate material in the aerosol. Continuous or near-continuous, near-real-time monitoring of the mass, for example in ambient air, can be accomplished by continuously or near-continuously sampling the ambient by pulling the particle carrier gas into and through the instrument view volume. The level of the detected optical signal is then at any time an instantaneous indicator of the mass of particles in the view volume.
Electronic signals suffer from what is known as “1/f noise”, contributed for example by the detector. Electronic noise, including the 1/f noise, can be removed or filtered by a technique involving modulation of the light source intensity in time. The amount of light scattered by the illuminated particles in the view volume will thus also be modulated away from a near-DC level. Synchronous detection of the modulated scattering is a sensitive method of separating the modulated scattering signal from the essentially steady level of electronic noise.
Unfortunately, the same detected optical signal also includes a background parasitic scattering from the nephelometer itself, such as scattering from internal optical components or from contaminants within the sensor instruments. Because this parasitic scattering will also be modulated in the same manner as that of the particle scattering, it cannot be separated out by the light modulation technique and forms a background noise level in the instrument underlying the desired particle scatter signal.
What is needed is a nephelometer instrument and noise filtering technique that can not only remove the 1/f or electronic noise, but also the background parasitic scattering contributions of the instrument, so as to obtain a scattering signal that is a more accurate indicator of ambient particle concentration.